The invention relates to a process for the preparation of prilled urea type additive for maize silage. Prills are globular particles, which are obtained in a known manner by spraying a melt, such as a urea or fertilizer melt, in the form of droplets from a perforated, rapidly rotating reservoir under the influence of the reservoir's centrifugal force, and thereafter allowing the falling droplets to cool down and solidify to form very uniform particles of a globular shape.
Maize silage is a cattle feed obtained by mowing the maize while green, then cutting the mown maize, and preserving it in a silo. The ensiled product contains too little protein relative to its starch content and, according to known procedures to correct this deficiency it is customary, before ensiling the product, to add urea to the maize during the cutting operation. It is recommended if the maize has a dry matter content of 30 % by weight, to add the urea in an amount by weight equalling 0.5 % calculated to the weight of maize.
However, compared with grass, maize conditioned with urea is also deficient in mineral constituents. In fact, according to a publication by a French cattle-breeding experimental station, the Institut Technique de l'Elevage Bovin, at Paris, this deficiency can be remedied by adding to the maize, besides urea, a mineral additive mixture, as a supplement to adjust the mineral content of the urea treated maize to compare favorable with grass. The mineral additive mixture consists essentially of:
______________________________________ dicalcium phosphate 60 % wt sodium chloride (iodized) 20 % wt sodium sulphate 10 % wt magnesium sulphate 6.80 % wt ______________________________________
And the remainder consists of compounds of trace elements in the form of sulphates and the like such as:
______________________________________ zinc sulphate 1.00 % wt iron sulphate 1.50 % wt manganese sulphate 0.30 % wt copper sulphate 0.40 % wt cobalt sulphate 0.01 % wt ______________________________________
The trace elements may also be added in the form of oxides or carbonates, instead of sulphates. In that case, a mixture having more or less the following composition may, for instance, be used.
______________________________________ dicalcium phosphate 60 % wt sodium chloride (iodized) 20 % wt magnesium sulphate 10.4 % wt (kieserite) sodium sulphate 8.7 % wt iron carbonate 0.52 % wt zinc oxide 0.15 % wt cupric oxide 0.13 % wt manganese oxide 0.10 % wt cobalt oxide or 0.003 % wt cobalt carbonate ______________________________________
There is commercially available a granulated additive for maize silage which contains urea and mineral constituents in 1:1 weight ratio; as with urea it is supplied to the maize during cutting in an amount of 10 kgs per ton of maize if the dry-matter content of the maize is 30 %.
Unlike the mineral mixture recommended by the above experimental station, the commercial product, whose composition is largely identical to the recommended one, contains no magnesium sulphate, but magnesium carbonate in the form of dolomite; also it does not contain sodium chloride. Further, the commercial product consists of granules made by a technique other than the prilling process.
While prilling is not the only form of making the granules, on the other hand it is attractive for a urea manufacturer already having an existing plant to prepare a prilled additive on the basis of urea and mineral constituents in the same plant and equipment as is used for production of all-urea prills. The problem, however, is that, unlike a melt consisting of 100 % urea, a suspension made of a mixture based on about 50 % weight of urea and about 50 % wt. of the above mentioned mineral composition cannot simply be prilled in a tower built for the manufacture of urea prills, the height of fall of the tower being too small for the purpose.
As a general rule it can be stated that the preparation of urea prills with an average diameter of 1.7 mm requires a height of fall of 30 m; during their fall, the sprayed urea droplets cool down from 130.degree. C to approximately 70.degree. C, at which temperature they are sufficiently hard and deformationresistant. However, in contrast to the experience gained in the preparation of urea prills, it has been found that prills of the same size made by spraying a suspension of about 50 % weight of urea and about 50 % weight of minerals of the above composition are still soft at 70.degree. C. Thus the addition of the minerals appears to have a considerable lowering effect on the crystallization point of the mixture, which implies that -- if the prills are to cool down further and solidify -- a much greater height of fall is required. We have found that the lowering of the crystallization point is caused mainly by sodium chloride dissolving in the urea melt and of the mineral constituents added to the urea, by far the largest portion -- dicalcium phosphate and magnesium sulphate -- remains in suspension. The sodium chloride component, which accounts for about 20 % of the total of mineral constituents, dissolves to a large part, and forms a eutectic mixture with the urea.
Accordingly it is an object of the present invention to provide a process for the preparation of a urea-based maize additive which also contains needed trace minerals in the form of prills using conventional urea prilling equipment.